1. Field of the Invention
This invention relates to a dielectric sheath for electrodes of corona pre-treatment installations comprising a grounded cylinder and electrodes with electrode mounts which maintain the electrodes at a defined distance from the grounded cylinder.
2. Description of Prior Art
It is necessary to pre-treat the surface of plastic foils and labels made therefrom, so that they can be permanently imprinted. This is achieved by so-called corona pre-treatment installations which change the molecular structure of the surface. Installations of this type are known in various forms. They consist of different components, namely a high frequency generator, a high voltage transformer, a grounded cylinder over which the foil of the labels which are to be treated is guided, and a plurality of electrodes distributed around the periphery of the grounded cylinder.
Although installations of this type have proven themselves over a long period of time, there are some problems in connection with them. One problem is that the space for the component of the corona pre-treatment installation which comprises the grounded cylinder and the electrodes is extremely limited.
For this reason, it is desired, on the one hand, to obtain as great a width as possible for treatment and, on the other hand, to keep the total width of the components as narrow as possible.
Up to now, this has not been satisfactorily achieved. The reasons for this are the electrodes and their sheaths which form the dielectric. Customarily, such electrodes have consisted of metal bars which were replaceably fastened on appropriate mountings and which were sheathed in a silicon hose. Because comparatively high temperatures are generated in the course of the high frequency discharges between the electrodes and the grounded cylinder, it is necessary to rotate the electrodes continuously during operation. Of course, this requires a relatively extensive mechanical outlay. Nevertheless, the service life of the dielectric, i.e. the silicon hose, is very limited. As a result, it is necessary to unscrew and replace the electrodes so that they can be sent to the supplier for providing them with a fresh insulating sheath.
The sheathed electrodes are relatively expensive and accordingly the user often only stores a small amount. As a result, if replacement electrodes are lacking, there are often days-long work interruptions.
Another problem with the known sheathed electrodes is that the silicon hose itself contains insufficient moisture, thus requiring that the electrodes be longer than the sheath.
However, this necessarily means that the electrodes must be longer than the grounded cylinder and extend on both sides beyond the grounded cylinder so that no short circuit voltage breakdown between the non-insulated ends of the electrode and the grounded cylinder can occur. A natural result of this is either a greater width of the component of the corona pre-treatment installation or a shorter grounded cylinder, which in turn leads to a reduction of the processing width.